The use of dielectric filters to selectively attenuate signals for electronic applications is well known in the art. Typically, this is accomplished using dielectric materials having metallized through holes formed therein to create one quarter wavelength resonators. The use of guard holes (also referred to as ground holes) to decouple consecutive resonators is also known in the art. FIG. 1 shows a prior art dielectric ceramic block filter 100 having guard holes 104 strategically placed between the resonators 102 to decouple the adjacent resonators. Heretofore, the guard holes have always been shorted (electrically connected to ground) at both the top and bottom surfaces of the filter block to achieve the desired electrical results. Shorting to ground is achieved by creating a continuously conductive path from the guard holes to the metallization on the sidewalls of the filter block. Also, prior art band reject filters have only provided rejection on either the high side or the low side of the passband.
Moreover, dielectric ceramic block filters have often been used as "stand alone" components that need to be integrated into the circuit board designs of a cellular telephone, for example. A bandstop filter module offers the advantages of additional integration by incorporating the dielectric monolithic block along with a circuit board, transmission lines, coil inductors and a shield to form the filter module.
Such a package offers the advantages of a small, compact, low-profile package which can be customized to meet specific radio architecture applications such as a duplexer, triplexer, or dual band filter module. Additionally, as more design features are placed directly into the filter module, less work is required by those designing the rest of the circuit board. Thus, a veritable "drop in" component can be designed which can perform elaborate filtering functions and requires only a predetermined footprint on the master circuit board.
A bandstop filter module which is designed to offer a shunt zero which creates a notch or a sharp point of increased attenuation in the frequency response and which is achieved by selectively leaving regions around the guard holes unmetallized on the top surface of a dielectric block, and where the bandstop filter module offers the advantages of additional integration in a low profile, small, compact, customized filter package, would be considered an improvement in the art.